It's simply a matter of abstraction, like having bills addressed to "The Club Secretary" instead of "Susan". If you have bills sent to susan, and sales catalogs, and CVs, then what happens if you hire Bob to handle HR stuff like CVs? The mail is still addressed to Susan, and no one will know it's for Bob until Susan has found time to open it and make sure it's not meant for her. So instead of naming "Susan", the actual person, you name "Human Resource Department" -- the ROLE.

Likewise, you name computers for their ROLEs, not the actual computers. So you have a computer called if you serve websites, and a computer called ftp.yourcompany.com if you provide an FTP service, etc. If you don't, then one computer, yourcompany.com, has to receive all the internet traffic, and then pass it on to the right place. could be all the webservers at google, but if yourcompany.com is a laptop, the laptop will be overloaded, while the web servers will still be sitting around waiting for information to reach them. Like Susan handling all the mail initially, one computer can handle all the roles initially, but the separate computer/domain names help to separate (or consolidate) things as necessary.